The present invention is in the field of aluminum extraction and reduction and particularly relates to a direct-reduction process for producing aluminum from a raw or natural aluminum silicate ore such as kyanite and sillimanite.
For over 80 years, aluminum has been produced by the twopart Bayer-Hall process, wherein alumina (Al.sub.2 O.sub.3) is first extracted from bauxite ore and the alumina is then electrolytically reduced in molten cryolite (sodium aluminum fluorides) to free aluminum metal. Although the process has been highly successful commercially, it consumes large quantities of electricity and takes about four pounds of bauxite to produce one pound of aluminum. Bauxite comprises 45 to 60 percent aluminum oxide, 3 to 25 percent iron oxide, 2.5 to 18 percent silicon oxide, 2 to 5 percent titanium oxide, up to one percent other impurities, combined with 12 to 30 percent "water of crystallization." The ore varies greatly in the proportions of its constituents, and in color and consistency. Gibbsite, boehmite and diaspore are the hydrated aluminum oxide minerals normally found in bauxite.
The Bayer process for producing alumina basically involves a caustic leach at elevated temperature and pressure, followed by separation of the resulting sodium aluminate solution, and selective precipitation of the alumina. There are two principal variations of the process: (1) The European Bayer, in which the approximate conditions of leaching are at a pressure of 210 pounds per square inch, a temperature of 390.degree.F, a caustic concentration of 400 grams per liter, and a digestion time of 2 to 8 hours to effect solution of the monohydrate mineral boehmite; and (2) The American Bayer, in which a pressure of about 60 pounds per square inch, a temperature of about 290.degree.F, a caustic concentration of 170 grams per liter, and a digestion time of one-half to 1 hour are used to dissolve the trihydrate mineral gibbsite. In both processes, the pregnant solution is separated from the red mud tailings by countercurrent decantation and filtration. The liquor is cooled until it becomes supersaturated, then seeded with crystals of aluminum trihydrate. About one-half of the alumina in solution is precipitated in a 36 to 96 hour period. The precipitate is then filtered, washed and calcined at 2000.degree.F to obtain the final product. Caustic soda is regenerated in the precipitation step and, together with the unprecipitated alumina, is recycled to the digesters.
The finely divided residue resulting from leaching contains Fe.sub.2 O.sub.3, TiO.sub.2 and a complex sodium aluminum silicate compound, the latter representing a loss of soda and alumina. The quantity discarded in the residue is related to the silica content of the bauxite. Approximately 1.1 units of alumina and 1.2 units of soda are lost for each unit of silica in the ore. For economic treatment, the bauxite must contain less that 8 percent silica. Approximately 4 long dry tons of bauxite are required to produce two short tons of alumina, which upon electrolysis yields slightly more than 1 short ton of aluminum. In addition to bauxite, the Bayer process requires soda ash, lime for causticizing the soda ash and fuel oil, gas or coal.
Some modifications of the Bayer-Hall process have been made in order to utilize bauxite ores containing 12 to 15 percent silica. In one such process the ore is first subjected to a Bayer leach. The resulting red mud, which contains a complex sodium aluminum silicate compound, is sintered with limestone and soda ash, then leached with water to recover alumina and soda. The brown mud residue has a composition, on a dry basis, somewhat similar to that of portland cement. This process requires additional costs in capital investment, raw materials and processing, and the upper limit of silica for use in the process is about 15 percent.
The average grade of bauxite ore used in the Bayer-Hall process has continually declined. In 1930 ore used in the U.S. averaged 60 percent alumina and by 1963, the average was less than 50 percent alumina. Although it is anticipated that this average will decrease to about 35 percent alumina in the future, the process is generally limited to the use of bauxite ore high in aluminum content. Domestic reserves of such high grade are totally inadequate to meet current production requirements.
Another disadvantage of the Bayer-Hall process is its necessity for an adequate, dependable and long-range supply of alumina requiring discovery of new sources of raw materials and the solution of numerous mining and metallurgical problems. Problems of the process include the need for improving efficiency and development of methods for utilizing tailings. Mechanical beneficiation of low-grade bauxites is hampered by the high loss of alumina in removing iron and silica. A need therefore exists for a direct reduction process that frees aluminum from crude feed material and which material is readily available.
In another process, alumina is extracted commercially from high-iron bauxites by the Pedersen smelting process. In this process, bauxite, limestone, coke and iron ore are smelted in an electric furnace to produce pig iron and a calcium aluminate slag containing 30 to 50 percent alumina. The slag is leached with sodium carbonate solution, and the alumina trihydrate is precipitated by carbon dioxide.
One prior art direct reduction process for producing alumina has achieved some success in the laboratory, but has failed to achieve real commercial sucess. In this process, aluminum-containing metal feed, e.g., bauxite reduced with coke, is brought into contact at an elevated temperature with gaseous AlCl.sub.3 (or the tribromide) and the gaseous subhalide (monochloride or monobromide) is cooled in a separate zone to break the gas down to aluminum trihalide and purified aluminum. Aluminum is recovered in a molten, substantially pure state. The aluminum trihalide is recirculated to produce additional mono-halide. Severe temperature conditions, problems of handling hot metal, and the corrosive nature of the gases create many difficulties in operating the process.
In another direct reduction process, bauxite is partially reduced with carbon in an electric furnace, then it is further reduced with carbon to produce a mixture of aluminum and aluminum carbides. The aluminum is separated and the aluminum carbide recycled. Little or no commercial success has been achieved with this process.
Many other methods of recovering aluminum have been proposed, none of which have been particularly successful. Such processes include the treatment of alumina with aluminum sulfide and carbon at an elevated temperature; hydrogen reduction of alumina at above 100 atmospheres and above 400.degree.C; reaction between alumina and aluminum carbide at 1,980.degree.C; and electrolytic reduction of complex organoaluminum compounds such as NaF.sup.. 2Al(C.sub.2 H.sub.5).sub.3.
Another process comprises chlorinating alumina containing materials in a reactor to yield aluminum trichloride and reacting the aluminum trichloride with manganese to yield aluminum and manganese chloride.
A process for carbothermic production of aluminum from aluminum oxide is disclosed in U.S. Pat. No. 3,607,221.
Direct smelting of aluminum-silicon alloys from clay has been investigated. High-purity clay is used to minimize contamination of the alloy by iron and titanium. An electric furnace has been used with a carbon reductant, which may be coke, charcoal, sawdust, hogged fuel, or mixtures of these materials. At operating temperature, pure aluminum would volatize and react with oxides of carbon. This is prevented by the presence of silicon which alloys with the aluminum and reduces the amount of aluminum vapors that are produced. Further, the silicon preferentially reacts with any carbon which dissolves in the aluminum silicon alloy and prevents the formation of aluminum carbide which would be non-reactive in aluminum recovery operations.
Methods for recovering commercial-grade aluminum from aluminum-silicon alloys have also been investigated. Experimental procedures have included leaching the alloy with a molten metal such as zinc in which the aluminum dissolves and the silicon and impurities are relatively insoluble. The zinc is then distilled from the aluminum. In the subhalide process, a crude aluminum alloy is treated with AlCl.sub.3 at approximately 1000.degree.C to produce AlCl. The reaction is reversed by lowering the temperature; pure aluminum condenses and the AlCl.sub.3 vapors are recycled.
The present invention is particularly adapted to overcome the disadvantages, problems and difficulties of these prior art processes.
It is a primary object of the present invention to provide a complete direct reduction process for producing aluminum from a natural or raw ore, such a kyanite, which is available domestically in commercial quantities.
Another object of the present invention is to provide a process for producing substantially pure aluminum which is more economical than prior art processes.
Still another object of the present invention is to provide a process for producing aluminum wherein little or none of the materials used therein is lost in processing
A further object of the instant invention is to provide a new direct reduction process for aluminum which also provides a ferro-silicon alloy as a second principal product thereof.
Other objects and advantages of the invention will be readily apparent from a consideration of the description and drawings hereinafter.